In the Days of Hot Aluminum

Considering the 4.5 billion years or so that our solar system has been around, the whole thing seems to have formed in a hurry. Many astronomers think the cloud of dust and gas from which the planets began to coalesce lasted less than 500,000 years and that the entire process of planet formation was well under way a couple of million years after that. Now a team of researchers looking at the elements contained in some rather ordinary meteorites has found evidence that the primordial solar cloud survived not for 500,000 years but most likely for a few million, meaning that planets probably formed much later than anyone had thought.

One of the components of the solar nebula was a radioactive isotope of aluminum that may have helped heat up the interiors of the first large bodies in the solar system. Glenn MacPherson of the Smithsonian was interested in using some common aluminum-rich meteorites as a way of determining whether the solar nebula did indeed persist for some 500,000 years.

MacPherson looked at nine aluminum pockets contained in four garden-variety meteorites called chondrites. Since radioactive aluminum has a half-life of 730,000 years and decays into a heavy isotope of magnesium, MacPherson checked to see if the aluminum pockets contained any telltale heavy magnesium. The amount of such magnesium in the pockets of ordinary aluminum told MacPherson how much radioactive aluminum the pocket once held. If he found pockets rich in magnesium, they must have formed when there was still a lot of hot aluminum to go around. Likewise, magnesium- poor pockets would have formed at a later time, when the radioactive aluminum had been depleted in the solar nebula.

Most astronomers would have expected MacPherson to find one case or the other, since the solar nebula in which those pockets formed was thought to have lasted for only a relatively brief period. Instead he found a range of aluminum-magnesium ratios in amounts that suggested the magnesium pockets must have formed over some 2 to 5 million years, stretching from a hot-aluminum-rich era to a time when most of the hot aluminum in the solar dust cloud had already decayed.

The disk of gas and dust in which those pockets formed would, of course, also have to be at least that old, a find that significantly alters astronomersí timetable of events in the early solar system. Says MacPherson: ìThis means the planets formed pretty darn late.î